This invention relates to a time division multiplex communication system, and more particularly a time division multiplex communication system wherein a plurality of local offices distributed in a wide area and a center office controlling or supervising the local offices are interconnected by a loop transmission line pair so as to transmit time division multiplexed signals through the transmission line pair thereby enabling communication between the center office and respective local offices. In such communication system utilizing the loop transmission line pair, as the pair can be used in common by several offices, the efficiency of utilization of the line is high, so that this system is becoming important in the communication system. A typical one of such system is disclosed in Japanese Publication No. 32244/1976 titled "Electrical Communication System".
As shown in FIG. 1A of the accompanying drawing, according to the system disclosed therein, time division multiplexed signals are transmitted unidirectionally through a loop main transmission line MAIN as shown by arrows. Multiplexers LMX.sub.l -LMX.sub.n, which introduce and extract a pulse shaped signal into and out of a specific time slot among a plurality of time slots provide for the time division multiplexed signals that are introduced into the main transmission line. Therefore, when a communication is to be made between the multiplexers LMX.sub.l and LMX.sub.m, a specific time slot TS.sub.a is used in these multiplexers to transmit a signal from the multiplexer LMX.sub.l in the counterclockwise direction and to transmit a signal from the multiplexer LMX.sub.m also in the counterclockwise direction. Thus, the transmission of signals is performed around the main transmission line MAIN, that is unidirectionally.
For this reason, when a fault occurs in the main transmission line MAIN, the communication becomes impossible. To eliminate this difficulty, an auxiliary line AUXI, which transmits signals in the opposite direction, is installed in parallel to the main transmission line MAIN as shown in FIG. 1B. When a section between the multiplexers LMX.sub.m and LMX.sub.n becomes faulty at a point indicated by X, the main transmission lina MAIN and the auxiliary transmission line AUXI are interconnected at that faulty section to resume a loop shaped transmission line thus ensuring communication.
In the system shown in FIG. 1B, however, as the auxiliary transmission line AUXI is not normally used, it is not economical from the standpoint of installation. Moreover, as the auxiliary transmission line AUXI must be installed as a unidirection transmission line as in FIG. 1A, when constructing communication lines of large capacity cost are increased further.